A variety of techniques have been employed to deliver nutrients to growing plants and to extend or delay release of nutrients from the fertilizers. Conventional rapid-release fertilizers generally provide nutrients in the form of water-soluble salts. As a result, nutrients are rapidly available to plants, but also, in some instances, are rapidly leached out of the root zone into ground and surface waters. This loss of nutrients is an economic concern to the grower as well as an environmental concern to communities and ecosystems located downstream who rely on clean sources of water. To minimize nutrient losses, conventional fertilizers are typically applied in small doses multiple times during a typical growing season. Alternatively, controlled-release fertilizers, which employ various methods to slow the release of the water-soluble form of the nutrient, can be applied in a single large application, but these fertilizers are significantly more expensive than conventional fertilizers.
More nitrogen is applied as fertilizer than all other plant nutrients combined, and roughly half of all nitrogen applied globally is not taken up by plants and thus wasted. The dominant form of conventional nitrogen fertilizer is urea; other common forms include anhydrous ammonia, ammonium sulfate, and ammonium nitrate. These compounds either contain or release ammonium ions, which are readily retained by soils. However, soil microbial activity converts ammonium to nitrate, which is not retained by soils and thus easily lost by leaching processes. Nitrogen can also be lost from soils by volatilization of ammonia gas under high pH conditions, and by denitrification processes that yield dinitrogen, nitric oxide, and nitrous oxide gases, the latter of which is a powerful greenhouse gas. Thus, there are many reasons, both economic and environmental, to improve the efficiency of nitrogen usage in agriculture. One promising way is by the use of controlled-release fertilizers timed to provide nitrogen when the plant needs it.
The controlled-release fertilizers currently available typically employ ammonium salts or urea encased by a physical or chemical barrier to slow the solubilization of the fertilizer by water and its subsequent diffusion into the soil. A typical physical barrier is a porous polymer film; sulfur coatings are commonly used as chemical barriers.
In the early part of the twentieth century, aluminum nitride was proposed as a nitrogen fertilizer. Without further modification, this nitride reacts with dissolved silica, alumina, or aluminosilicate ions, which are abundant in soils, to form surface coatings that block access to the surface by water and thus prevent release of a large fraction of the nitrogen it contains. This reaction is one of the reasons for the current dominance of readily soluble ammonium-based fertilizers manufactured by the Haber-Bosch process.
Fly ash is a class of byproducts obtained from the combustion of coal that contain mixtures of various solid oxides, including Si, Al, and Ca. Roughly half of the fly ash that is produced finds other uses, such as substituting for Portland cement in concrete; the remainder is buried in landfills or storage ponds. The oxides in fly ash can be converted to Si, Al, and Ca nitrides by carbothermonitridation or by direct reaction with ammonia under high temperatures. As with pure aluminum nitride, without further treatment these nitrides develop surface coatings that prevent them from releasing a large fraction of their nitrogen in soils. Moreover, the fraction of nitrogen that is released by these untreated nitrides does not follow a near-linear rate to match crop demand for nitrogen, a necessary criterion for a successful slow-release fertilizer. Thus, despite the strong need for new and better slow-release nitrogen fertilizers, to date, solid nitrides have not been seen as practical alternatives. New compositions of fertilizers are needed that do not rely on soluble ammonium salts, provide controlled-release of nitrogen from the fertilizer, and minimize negative effects on the environment. The present invention addresses these needs.